Simulations of fluid hydrogen: Comparison of a dissociation model with tight-binding molecular dynamics

We compare the results of two complementary approaches, tight-binding molecular-dynamics simulations and a dissociation model, for determining the characteristics of dense, fluid hydrogen at pressures extending to megabars and temperatures to 10 000 K. Two tight-binding models were examined: one parametrization emphasized crystalline, molecular, and fluid properties, the other focused more on the intricate molecular interactions involving up to four hydrogen atoms. The two tight-binding cases and the dissociation model agree reasonably well for a variety of properties, including the equation of state, dissociation degree, and proton pair-correlation functions. In simulations of recently reported laser-driven shock experiments, the tight-binding and dissociative models predict different maximum compressions of four and five, respectively. We discuss the sensitivities of the models as well as give estimates for the region of validity of the chemical picture (dissociation model) and the accuracy of the dynamical picture (tight-binding simulations) in cases where molecular hydrogen still dominates the physical behavior.